The data doesn't live on Xbox One or Xbox 360, it lives on Bungie servers, connected to your Xbox Live account.
So yes, you can go back and forth, but it is not a transfer.
As such, all you have to do is start the game on either of the two platforms and your characters and everything will be there, ready to play.
This is also true for PS3 and PS4, the characters and your Destiny data is tied to your PSN account, on Bungie servers.
You do need to have purchased the game on both console generations though, so in order to play on both Xbox 360 and Xbox One you need to purchase the game for both Xbox 360 and Xbox One. Same goes for PS3 and PS4, you need to purchase it for both PS3 and PS4.
Whether you have a digital game downloaded from the respective store, or a store-bought game on disc doesn't matter one bit as the data is stored on Bungie servers tied to your account. This is not necessarily true for other games, but for Destiny it is. You can even mix and match, having a disc-based Destiny for Xbox 360, and a digital only Destiny for Xbox One.